Detection of ribcage boundary from digital chest image

ABSTRACT

To provide more correct ribcage boundary information and landmark information, the size of a searching ROI mapped on a digital chest image is decided, the ROI is set on the image, an image existing within the region set on the image is enhanced, a profile of pixel values existing within the region is obtained, and the obtained profile is analyzed so that candidate points for a ribcage boundary is obtained. These deciding, setting, enhancing, obtaining, and analyzing steps are repeated until a center of each searching ROI advances immediately before a given search limit. As a result, a series of candidate points for the ribcage boundary is provided.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to an image processing technique for automatically and accurately detecting an anatomical configuration from a (PA:postero-anterior) digitalized chest radiograph, which is necessary for computer-aided diagnosis of the chest, and in particular, to an image processing technique that makes it possible to, with higher precision, detect the boundary of a ribcage from a digitalized chest radiograph.

[0002] Digitized chest radiograph have been used widely in the field of computer-aided diagnosis. There have been known a wide variety of types of computer-aided diagnosis capable of automatically detecting ribcage boundary information and landmark information both specifying anatomical configurations of the chest. One conventional technique is provided by “Xin-Wei Xu and Kunio Doi, Image feature analysis for computer-aided diagnosis: Accurate determination of ribcage boundary in chest radiographs, Med. Phys. 22(5), May 1995.” This technique is also provided by Japanese Patent Laid-open Publication NO.7-37074.

[0003] This diagnostic technique uses lesion-enhanced images in order to detect temporal changes of diseases such as lung diseases among digitalized chest radiograph acquired at different times for the same patient's region. To raise diagnostic accuracy, this technique comprises the steps of obtaining a previous and current digital chest images, positioning both previous and current digital images by performing non-linear warping processing based on a non-linear warping technique on either the previous or current digital image, and making a subtraction between previous and current image one of which has undergone the non-linear warping. The non-linear warping technique uses information detected from a chest image in relation to its anatomical structure and is based on a local matching technique to be applied to a number of tiny regions of interest (ROIs) selected on the basis of the information. The non-linear warping technique is mapping of amounts of matching shift obtained between corresponding locations in two frames of images. The mapping is realized using amounts of local matching resulting from a local matching technique applied to the locations and a weighted fitting technique that uses weights resulting from image data analysis applied to the ROIs. In addition, the mapping of shift amounts is based on two-dimensional polynomial functions fitted to shift values.

[0004] However, the above conventional automatic detection technique tends to fail in detection of boundary candidate points if contrast is low in the vicinity of a ribcage boundary. If such a failure occurs, the ribcage boundary, which is acquired by approximating the boundary candidate points to polynomials, deviates from the true ribcage boundary. This reduces accuracy in computer-aided diagnosis as a whole.

[0005] In addition, when a searching ROI is designated with a diaphragm included in the region, the boundary candidate points are pulled toward structures within the lung views. The boundary candidate points are difficult to be detected on the true ribcage boundary. This results in lowered accuracy in computer-aided diagnosis as well.

[0006] Further, in cases a top lung is depicted in high contrast on a chest image, the detected boundary candidate points are pulled toward the ribs, clavicles, lesions, and/or others residing within the lung views. In this case, upper ribcage boundary candidate points will not be detected on the true upper ribcage boundary, thereby leading to lowered accuracy in computer-aided diagnosis.

[0007] Still further, if a searching ROI to search an upper ribcage boundary is designated so as not to cross the true ribcage boundary, it results in that boundaries of chest structures (the ribs, clavicles, lesions, scapulas, and/or others) other than the original true boundary are detected. This also deteriorates computer-aided diagnosis in accuracy.

[0008] For using features of a chest image and positional information indicative of an anatomical structure in computer-aided diagnosis, it is significant to acquire more accurate information about a ribcage boundary and landmarks. It has therefore been strongly desired that the more accurate information be available for computer-aided diagnosis.

SUMMARY OF THE INVENTION

[0009] An object of the present invention is to acquire both ribcage boundary information and landmark information with a higher precision, which has not been obtained so far, so that computer-aided diagnosis is able to gain increased accuracy of diagnosis.

[0010] In order to realize the object, as one aspect of the present invention, a method for detecting from a digital chest image a ribcage boundary for computer-aided diagnosis with which anatomical structure information is detected from the digital chest image, including repetitions each comprising the steps of: deciding a size of a searching ROI mapped on the digital chest image, the searching ROI being deferent in position repetition by repetition; setting the searching ROI on the digital chest image; enhancing an image within the searching ROI set on the digital chest image; obtaining a profile of pixel values existing within the searching ROI; and analyzing the obtained profile so as to decide a candidate point for the ribcage boundary, wherein the deciding, setting, enhancing, obtaining, and analyzing Steps are repeated until a center of each searching ROI advances immediately before a given search limit, thereby a series of candidate points for the ribcage boundary being provided. Using this method makes it possible to, with higher precision, detect correct ribcage boundaries from chest images which are difficult to be detected by the conventional technique.

[0011] Preferably, a step of lowering noise of the image residing in the searching ROI intervenes between the enhancing and obtaining steps. Thus, a correct ribcage boundary can be detected at a greater percentage even in the vicinity of the ribcage boundary which is low in contrast.

[0012] Still preferably, a step of weighting the profile of pixel values intervenes between the obtaining and analyzing Steps, wherein the profile consists of a horizontal pixel-value averaged vertical profile and a vertical pixel-value averaged horizontal profile and a weight of the weighting becomes small little by little as the weighting advances from a center of each of the vertical and horizontal the profiles toward the insides of the lung views. Hence, obstacles, such as the rib, clavicle, scapula, diaphragm, and/or lesions, to detection of a ribcage boundary can be removed, and a correct ribcage boundary can be detected at a greater percentage.

[0013] It is preferred that the size of each of the plurality of searching ROIs mapped on the digital chest image through the repetitions are fixed and independent on locations of the searching ROIs. This makes it possible to set a searching ROI at locations which were difficult for the conventional technique to set the ROI. Therefore, points of ribcage boundary candidates are easier to search. Even if failed in detection at some points, a ribcage boundary can be fitted to its almost true shape, as long as one or more points are acquired on its true boundary.

[0014] As another aspect of the present invention, there is provided a system for detecting from a digital chest image a ribcage boundary for computer-aided diagnosis with which anatomical structure information is detected from the digital chest image, comprising: a landmark detector for detecting landmark information serving as a landmark in displaying a chest configuration depicted on the digital chest image; a candidate point series detector for detecting both of an upper ribcage boundary point series and right/left ribcage boundary candidate point series from image data of the digital chest image and the land mark information; a boundary point series combining unit for combining the upper ribcage boundary point series with the right/left ribcage boundary candidate point series so as to produce ribcage boundary information indicative of a contour of ribs of the chest; and an image displaying processor for displaying ribcage information made up of the landmark information and the ribcage boundary information on the image. Therefore, contrast of an image enclosed by a searching ROI can be enhanced in detection of the upper ribcage boundary candidate point series and the right/left ribcage boundary candidate point series. This causes the ribcage boundary to be noticeable, providing ribcage boundary information with precision. A rate of success in detection of the ribcage boundary is then improved.

[0015] Preferably, the system further comprises an element for weighting a pixel-value profile of an image residing within a searching ROI to be mapped on the digital chest image. Hence, influences of the rib, clavicle, scapula, diaphragm, and/or lesions can be avoided, so that more correct ribcage boundary information are provided. A rate of success in detection of the ribcage boundary is raises noticeably.

[0016] It is also preferred that the system further comprises an element for smoothing the image residing within the searching ROI by using a moving average filter composed of a region in area smaller than the searching ROI. In consequence, without being influenced by even a slight amount of noise, more correct ribcage boundary information are provided, so a rate of success in detection of the ribcage boundary is raises remarkably.

[0017] It is also preferred that the searching ROI is set to have a length larger than widths of the ribs and clavicles in the digital chest image, further comprising means for searching the points by using the searching ROI of which size is fixed from a search starting point to a search end point. This makes it possible to set a searching ROI at locations which were difficult for the conventional technique to set the ROI. Hence, points of ribcage boundary candidates are easier to search. Even if failed in detection at some points, a ribcage boundary can be fitted to its almost true shape, as long as one or more points are acquired on its true boundary.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] In the accompanying drawings:

[0019]FIG. 1 is a block diagram showing the entire configuration of a ribcage boundary detecting system for digital chest images according to an embodiment of the present invention;

[0020]FIG. 2 is an illustration of ribcage information treated as anatomical structure information of a chest image;

[0021]FIG. 3 is an illustration of landmark information composing part of the ribcage information;

[0022]FIG. 4 illustrates series of boundary candidate points adopted as ribcage boundary information composing part of the ribcage information;

[0023]FIG. 5 shows a flowchart relating to decision of upper ribcage boundary candidate point series and decision of right/left ribcage candidate point series, which falls in a method of detecting a ribcage boundary from a digital chest image, which is carried out by a ribcage boundary detecting system according to the embodiment of the present invention;

[0024]FIG. 6 illustrates a decided result of a series of lower-right ribcage boundary candidate points according to a conventional method;

[0025]FIG. 7 is a flowchart made by adding, into the flowchart of FIG. 5, a step for lowering noise of an image in each searching ROI;

[0026]FIG. 8 is a flowchart made by adding, into the flowchart of FIG. 7, a Step for weighting a density integrated average profile;

[0027]FIG. 9 is an illustration for exemplifying mapping of searching ROIs in detecting series of ribcage boundary candidate points;

[0028]FIG. 10A illustrates the relationship between a mapped position of a searching ROI in the vicinity of lower right ribcages and weighting a profile within the searching ROI;

[0029]FIG. 10B graphically shows a before-weighted profile;

[0030]FIG. 10C graphically shows an after-weighted profile;

[0031]FIG. 11A pictorially shows a misdetected example according to the conventional technique, which is a result detected from a ribcage boundary that exist in the lower lung sinister;

[0032]FIG. 11B pictorially shows a successfully detected example according to the present invention technique, which is a result detected from a ribcage boundary that exist in the lower lung sinister;

[0033]FIG. 12 details a flowchart based on the conventional technique in deciding a size of each searching ROI, which falls in decision of a series of boundary candidate points in deciding a series of upper ribcage candidate points;

[0034]FIG. 13 pictorially shows a misdetected ribcage boundary in a top lung end portion according to the conventional technique;

[0035]FIG. 14A pictorially exemplifies a misdetected upper ribcage boundary according to the conventional technique; and

[0036]FIG. 14B pictorially exemplifies a successfully detected upper ribcage boundary according to the present invention.

PREFERRED EMBODIMENTS OF THE INVENTION

[0037] Referring to the accompanying drawings, preferred embodiments of the present invention will now be described.

[0038] At first, referring to FIGS. 1 to 4, the configuration of a ribcage boundary detecting system according to an embodiment, which is used for detecting the boundary of a ribcage from a digital chest image, will now be described.

[0039]FIG. 1 shows the entire configuration of the ribcage boundary detecting system according to the present embodiment. FIG. 2 illustrates pieces of ribcage information adopted as anatomical structure information of a chest image. FIG. 3 illustrates pieces of landmark information which composes part of the pieces of ribcage information. FIG. 4 illustrates lines of boundary candidate points serving as pieces of ribcage boundary information which compose part of the pieces of ribcage information.

[0040] As shown in those figures, the ribcage information of a chest image consists of pieces of ribcage boundary information depicting contour lines connecting outer ends of the chest ribs and pieces of landmark information taken as landmarks for displaying a chest configuration.

[0041] As shown in FIG. 2, the ribcage boundary information is made up of an upper ribcage boundary E that connects series of upper ribcage boundary points positioned higher than a lung length 1/5 line, a right ribcage boundary F that connects series of right ribcage points positioned lower than the lung length 1/5 line, and a left ribcage boundary G that connects series of left ribcage points positioned lower than the lung length 1/5 line.

[0042] As shown in FIG. 3, the landmark information includes coordinates showing positions, such as a center H of the right lung on the lung length 1/5 line, an X-coordinate I of a midline on the lung length 1/5 line, a center J of the left lung on the lung length 1/5 line, a center K of the right lung on the lung length 1/2 line, an X-coordinate L of a midline on the lung length 1/2 line, and a center M of the left lung on the lung length 1/2 line; Y-coordinates, such as a top end line N of the lung, lung length 1/5 line O, lung length 1/2 line P, lung bottom line Q of the lung; and lengths such as a lung length R.

[0043] As shown in FIG. 4, the series of the boundary candidate points, which serve as the ribcage boundary information, include a series A of upper right ribcage boundary candidate points, a series B of upper left ribcage boundary candidate points, a series C of right ribcage boundary candidate points, a series D of left ribcage boundary candidate points, series A+B of upper ribcage boundary candidate points, and series C+D of left/right ribcage boundary candidate points.

[0044] In the image processing system of FIG. 1, there is provided a digital image inputting apparatus 1 capable of digitizing chest radiographs that were imaged. For digitizing a chest radiograph into a chest image (PA chest image), the digital image inputting apparatus 1 serves as an apparatus such as a film digitizer for digitizing a film. For imaging a chest radiograph in digital, the digital image inputting apparatus 1 operates as a digital imaging system such as a CR (computed radiography) system. Either system can be used, provided that it is able to digitize chest films.

[0045] An image data storing memory 2 is provided to memorize chest images digitized by the digital image inputting apparatus 1.

[0046] A landmark information detector 3, which receives chest frontal image data from the digital image inputting apparatus 1, detects information from the chest frontal image data the landmark composing part of the ribcage information.

[0047] An upper ribcage boundary point series detector 4, connected with both landmark information detector 3 and image data storing memory 2, detects upper ribcage boundary point series A+B from chest frontal image data and landmark information.

[0048] A right/left ribcage boundary candidate point series detector 5, connected with the upper ribcage boundary point series detector 4, landmark information detector 3, and image data storing memory 2, detects right/left ribcage boundary candidate point series from chest frontal image data, information about upper ribcage boundary candidate point series, and landmark information.

[0049] A curve fitting calculator 6 for upper ribcage boundary candidate point series, connected with both upper ribcage boundary point series detector 4 and image data storing memory 4, performs curve fitting calculation on the upper ribcage boundary point series obtained by the detector 4, thereby providing series of continuously connected points with respect to X-coordinates.

[0050] A curve fitting calculator 7 for rigtht/left ribcage boundary candidate point series, connected with both right/left ribcage boundary candidate point series detector 5 and image data storing memory 4, performs curve fitting calculation on the right lung/left lung ribcage boundary point series obtained by the detector 5, thereby providing lines of continuously connected points with respect to Y-coordinates.

[0051] A boundary point series combining unit 8, connected with both curve fitting calculators 6 and 7, combines the upper ribcage boundary point series and the right lung/left lung ribcage boundary point series on the basis of the landmark information, thereby obtaining a ribcage boundary point series for each of the right and left ribcages.

[0052] A ribcage information storing memory 9, which is provided to be connected with both landmark information detector 3 and boundary point series combining unit 8, memorizes the ribcage boundary information obtained by the combining unit 8 and the landmark information detected by the detector 3.

[0053] An image displaying processor 10, coupled with the ribcage information storing memory 9 and the image data storing memory 2, performs processing for displaying the ribcage boundary information, chest image data, and others. The processed image data by this processor 10 are then sent to a CRT (cathode ray tube) to visualize an image which can be observed by users.

[0054] Other display devices, such as a plasma display or liquid crystal display, can be used instead of the CRT 11. But it is preferable to use the CRT when taking a graduation display performance into account. Moreover, it is preferred that a CRT of which scanning line is more than 1000 in number, which is known as a highly fine CRT for medial diagnosis.

[0055] Referring to FIGS. 5 to 9, a method for detecting a ribcage boundary of a digital chest image in the present embodiment will now be described. This method is carried out using the ribcage boundary detecting system described above.

[0056] In this detection of a ribcage boundary, search starting points to decide ribcage boundaries of right/left lung fields are set to S1, S2, S3, and S4 as shown in FIG. 4. The point S1, which is an intersection made between the top end line N of a lung and a right lung angular line T, is handled as a search starting point for upper right ribcage boundary candidate points. The point S2, which is an intersection made between the top end line N and a left lung angular line U, is handled as a search starting point for upper left ribcage boundary candidate points. The point S3, which is located near a right ribcage boundary of the right lung on the lung length 1/2 line, is handled as a search starting point for right ribcage boundary candidate points. The point S4, which is located near a left ribcage boundary of the left lung on the lung length 1/2 line, is handled as a search starting point for left ribcage boundary candidate points.

[0057]FIG. 5 shows a flowchart for deciding series of upper ribcage boundary candidate points and series of right/left ribcage boundary candidate points from a chest image. This flowchart expresses the method for detecting a ribcage boundary, which is used by the ribcage boundary detecting system according to the present invention.

[0058] For deciding a center of each searching ROI by using digitized image data 12 (Step S1), the center will be first calculated based on a search starting point and a candidate point acquiring distance. In the second or later time of acquisition, which is repeated, the center of the searching ROI for the next candidate point is calculated on the basis of the center of the last-acquired searching ROI and the candidate point acquiring distance. When the center of a certain searching ROI exceeds a search limit point after deciding its center several times, the search will be terminated (Step S2).

[0059] In the present detection, the candidate point acquiring distance is set, as references, to a value of 1% of the entire image width for searching upper ribcage boundary candidate point series and to a value of 3% of the entire image length for right/left ribcage boundary candidate point series. The smaller the candidate point acquiring distance, the more time for processing it takes, but candidate points can be obtained at fine intervals. In contrast, the larger the candidate point acquiring distance, the less time for processing it takes, but candidate points obtained are rough in intervals. Therefore, it is preferred that the distance be determined with referring to the foregoing percentages, 1% and 3%, depending on the type of diagnosis.

[0060] Then, a size of the searching ROI at the search starting point for the upper ribcage boundary candidate point series and a size of the searching ROI at the search starting point for the right/left ribcage boundary candidate point series are decided (Step S3). The former size (width×length) is “M×N (M<N)” at the search starting point and “M×L (L<N)” at positions other than the search starting point, while the latter size is “U×V (U>V)” at the search starting point and “W×V (W<U)” at positions other than the search starting point. As a target of the size of the searching ROI, it is preferable that its height is larger than the widths of the ribs and clavicles. In the present detection, as examples for size references, M is set to a value of about 2% of the entire image width, N to a value of about 8% of the entire image length, L to a value of about 6% of the entire image length, U to a value of about 8% of the entire image width, V to a value of about 3% of the entire image length, and W to a value of about 6% of the entire image width. Alternatively, sizes formed by adding or reducing distance of several pieces of pixels to or from each of the above sizes are still available.

[0061] Then, the center of the searching ROI and a pixel value of a rectangular area shown in the size of the searching ROI, which were obtained at Step 1 to 3, are acquired on the image (Step S4).

[0062] On the basis of image data clipped at Step S4, contrast enhancing processing is performed on image data enclosed by the searching ROI, enabling an easy finding of boundary candidate points at the next process of Step S6 (Step S5).

[0063] In the conventional technique, as shown in (a) of FIG. 6 to which this Step S5 is not applied, candidate points for a lower right ribcage boundary are weakly contrasted. Consequently, when a lung structure such a diaphragm is once detected erroneously, this influences the location of the next searching ROI, resulting in that such misdetection will occur in succession. A boundary to be generated through curve fitting therefore becomes considerably different from its original boundary, as shown in (b) of FIG. 6. However, in the present detection, because processing of Step S5 is placed, such failures in the detection can be avoided, providing a correct ribcage boundary.

[0064] An integrated average profile of pixel values in the searching ROI, which is obtained at Step S5, is then acquired (Step S6). For a search of series of upper ribcage boundary candidate points, pixel values of M-pixels (about 2% of the image width) residing in the X-direction of the searching ROI are integrated and averaged, then this integration and averaging is carried out at each pixel belonging to the size in the Y-direction of the searching ROI, thus providing a pixel-value integrated average profile. For a search of series of right/left ribcage boundary candidate points, pixel values of V-pixels (about 3% of the image length) residing in the Y-direction of the searching ROI are integrated and averaged, then this integration and averaging is carried out at each pixel belonging to the size in the X-direction of the searching ROI, thus providing a pixel-value integrated average profile.

[0065] Then, profile data are smoothed in order to reduce noise of the pixel-value integrated average profiles acquired at Step S6 (Step S7).

[0066] Furthermore, a first and second derivative of each of the smoothed pixel-value integrated average profiles obtained at Step S7 are calculated for an analysis of each profile (Step S8).

[0067] Boundary candidate points are calculated from the first and second derivative obtained at Step S8 (Step S9). At this Step, found are locations where the first derivative equals zero, the first derivative is a maximum, and the second derivative equals a minimum, so that the candidate boundary points are obtained.

[0068] Repeating of the aforementioned Steps S1 to S9 a plurality of times leads to acquisition of the desired series of boundary candidate points.

[0069] Though contrast within the searching ROI may be raised by various techniques, such as histogram equalization, histogram transformation, gamma correction, and gray scale processing, it is most desirable, from the viewpoint of cost, to adopt the histogram flattening technique, which does not require calculation of particular coefficients.

[0070] There is provided one modification for the above detection. FIG. 7 is a flowchart showing the modification, in which a step for a noise reduction from an image residing in the searching ROI is added to the flowchart shown in FIG. 5 which explains both decision of the upper ribcage candidate boundary point series and the right/left ribcage boundary candidate point series.

[0071] There is a possibility that the contrast-enhanced image in the searching ROI acquired at Step S5 includes a slight amount of noise, such as the clavicle, the rib, and shade, in contrast to ribcage information. To improve this, a procedure to remove such noise by smoothing the image within the searching ROI is added next to Step S5 (Step S10).

[0072] Adoption of Step S10 subsequently to Step S5, as shown in FIG. 7, allows noise existing inside the lung field to be removed, largely reducing a misdetection rate of such objects as the clavicle, the rib, shade, or others from the lung field. This noise removal is performed so effectively that Step S10 may be adopted independently, without adopting Step S5.

[0073] A practical technique carried out at Step S10 is using a moving average filter with a matrix size of “m×n,” which removes noise of the image within the searching ROI. The matrix size “m x n” is connected to a size of “M×N” of the searching ROI through the relationships of M>m and N>n. Therefore, the larger m and n in value, the greater the noise reduction effect, while the smaller m and n, the less the noise reduction effect. The values of m and n can be selected suitably depending on a searched region. In the detection method according to the present invention, both values are set such that m=3 and n=3, regardless of locations of searched regions. Other smoothing filters may also be used selectively in the same condition.

[0074] Additionally, there is another modification for the foregoing detection. FIG. 8 is a flowchart showing the modification, in which a step for weighting the pixel-value integrated average profile is added to the flowchart shown in FIG. 7 which explains both decision of the upper ribcage candidate boundary point series and the right/left ribcage boundary candidate point series.

[0075] The weighting processing is performed, inside the searching ROI, on the pixel-value integrated average profile using weights made smaller than those used outside the searching ROI (Step S11). Such smaller weights are obtained based on lung-view characteristics of the chest (shapes, contrast of pixel data, etc.). This causes large noise objects, such as the clavicle, the rib, and shads, excluding the ribcage, to be removed from the profile obtained at Step S6.

[0076] A more practical technique carried out at Step S11 is shown in FIG. 9. As shown therein, a premise is made such that, for detecting the upper ribcage boundary candidate point series, there is noise (the clavicle, rib, diaphragm, and others) inside a horizontal pixel-value integrated average vertical profile obtained with a vertically rectangular searching ROI Ry, whilst for detecting the right/left ribcage boundary candidate point series, there is such noise inside a vertical pixel-value integrated average horizontal profile obtained with a laterally rectangular searching ROI Rx. Based on these premises, the weighting of the integrated average pixel values is performed according to the following equation.

PDst(i)=PSrc(i)×Weight

[0077] where

[0078] Weight=1.0 for (0≦i≦n−1); or

[0079] Weight=1−j/(N−n) for (n≦i≦N−1),

[0080] n:n<N, i: 0≦i≦N−1, j: 1≦j≦N−n, and N: width of profile,

[0081] PDst(i): profile value obtained after i-th weighting from the outside, and

[0082] PSrc(i): profile value obtained before i-th weighting from the outside.

[0083] As shown in the above equation, the weights gradually reduce as the weighting goes from a center of each of the vertical profile horizontal profile toward the inside of the ROI. As a result, in the right/left ribcage boundary search, the center of the next searching ROI is moved directly under from both the center of the last searching ROI around the right lower ribcage detected last time and detected candidate points, as an image of processing object, as shown in FIG. 10A. When performing weighting on the image-value integrated average profile, the profile before weighting shown in FIG. 10B changes in shape into the profile after weighting shown in FIG. 10C.

[0084] Though the above detection method adopts n=N/2, values of n and N can be selected appropriately depending on a size of the searching ROI and/or anatomical positions of the boundary candidate points to be detected. While this weighting method uses a linear function, non-linear function may be used in the weighting, as long as its weights reduce as the weighting goes toward the inside of the lung field.

[0085]FIG. 11A shows an example of misdetection occurring in the right lung lower-field ribcage boundary. In the conventional technique, if an searching ROI is placed on the diaphragm, a diaphragm boundary with higher contrast than the ribcage boundary is misdetected. This causes misdetection to occur one after another afterward, no correct ribcage being provided. In contrast, the detection according to the invention is executed by weighting the pixel-value integrated average profile produced from an image existing within a searching ROI, which makes it possible to remove the diaphragm which is inconvenient for detection of the ribcage boundary. Accordingly, as shown in FIG. 11B, candidate points for the true ribcage boundary can be obtained with precision.

[0086]FIG. 12 is a detailed flowchart showing a conventional technique for detecting the size of each searching ROI, which corresponds to Step S3 of FIG. 5, for example, assigned for detection of boundary candidate point series carried out in the processing of deciding upper lung ribcage candidate point series.

[0087] In the conventional region-size deciding technique, it is determined whether a searching ROI is arranged at a search starting point or not (Step S12). When it is determined that the region is located at the search starting point, the processing is made to proceed to Step S13 that gives “M×N” to the region size. If it is determined that the region is not located at the search starting point, the processing is made to proceed to Step S14 that gives “M×L (N>L)” to the region size. That is, in this conventional technique, the size of the searching ROI is set to “M×N” at the search starting point and to “M×L” at points other than the search starting point.

[0088] In contrast, in the searching method of the present invention, the size of a searching ROI is fixed at “M×N” over the entire search, regardless of whether or not the searching ROI is located at the search starting point.

[0089] In cases the conventional technique is used in which the size of the searching ROI is M×N at the search starting point and M×L at other locations, positions where a clavicle and a rib are overlapped each other are misdetected as the lung upper end, as shown in FIG. 13. This is because contrast inside the lung field is high due to an apparent difference in pixel value between the lungs and the clavicles, ribs, and others in the top lung field. However, as in the case of the detection of the present invention, the size of a searching ROI is fixed at M×N through the whole search, the searching ROI can be located at positions where it has not been located so far. This makes it easier to find upper lung ribcage boundary candidate points. Even when the detection is performed unsuccessfully, acquisition of one or more candidate points on the true upper lung ribcage boundary permits the upper ribcage boundary to be fitted to its true shape using with a fourth-order fitting curve. In this detection, the upper lung ribcage boundary candidate point series are detected on the condition that M=11 and N=47.

[0090] While the above-mentioned is the description about the decision of the upper lung ribcage boundary candidate point series, the same concept can be of course applied to the decision of the right/left ribcage boundary candidate point series. In the latter case, the relationship between M and N should be replaced with U and V.

[0091] The conventional technique led to a misdetection of the upper left ribcage boundary candidate points at incorrect positions (on the clavicle), as shown in FIG. 14A. This results from the fact that the candidate points are pulled toward the ribs or clavicles, because the top of the lung of the left lung field is high in contrast. In contrast, in the detection of the present invention, since being fixed at M×N over all the search, the size of a searching ROI can be located on the upper left ribcage boundary. Thus, as shown in FIG. 14B, at least one candidate point can be located on the true upper left ribcage boundary, and can undergo curve fitting. A boundary similar to the true upper left ribcage boundary was acquired.

[0092] Additionally, the system and method for detecting the ribcage boundary from a digital chest image according to the embodiment of the invention can be used for computer-aided diagnosis requiring lesion-enhanced images in order that temporal changes of lung diseases or the like are detected from plural digital chest images acquired from the same patient's region at different times, like the conventional technique. This system and method are also available for a single image. For example, it is possible to detect anatomical structure information from a single frame of chest image to utilize it for computer-aided diagnosis.

[0093] Using actual images of 194 pieces (97 sets), the inventors made an experiment to know a rate of success in detecting ribcage boundary information (that is, probability that no misdetection is caused in an image). In the case of the conventional technique, as shown in Table 1, the rate of success was 27.6%. In contrast, in the technique to which the present invention is applied, where the image data existing within a searching ROI is enhanced in contrast, noise-reduced, and weighted to their pixel-value integrated average profile, the size of each searching ROI being fixed at “M×N” for all the search, the rate of success was improved up to 94.8%, as shown in Table 2. Especially, the perfect rate of success, 100%, was obtained for the right/left lower ribcage boundaries. TABLE 1 (Conventional technique) SUCCESS/FAILURE NUMBER OF EVENTS NO. IN DETECTION (PERCENTAGE) 1 success  53(=27.6%) 2 failure 141(=72.4%) 3 classi- failure in lower 60(=30.9%)  99(=61.3%) fication right ribcage 4 failure in lower 39(=20.1%) left ribcage 5 failure in upper 61(=31.4%) 119(=61.3%) right ribcage 6 failure in upper 58(=29.9%) left ribcage 7 failure in upper- 6(=3.1%) most end of lung 8 inaccuracy in  3(=1.54%)  8(=4.12%) right connecting part 9 inaccuracy in  5(=2.57%) left connecting part 10 failure in lower-  1(=0.51%)  1(=0.51%) most end of lung

[0094] TABLE 2 (The present invention technique) SUCCESS/FAILURE NUMBER OF EVENTS NO. IN DETECTION (PERCENTAGE) 1 success 184(=94.8%) 2 failure  10(=5.02%) 3 classi- failure in lower 0(=0.00%)  0(=0.00%) fication right ribcage 4 failure in lower 0(=0.00%) left ribcage 5 failure in upper 7(=3.60%) 13(=6.70%) right ribcage 6 failure in upper 6(=3.10%) left ribcage 7 failure in upper- 6(=3.1%) most end of lung 8 inaccuracy in 1(=0.67%)  2(=1.03%) right connecting part 9 inaccuracy in 1(=0.67%) left connecting part

[0095] Although the description above contains many specifities, these should not be construed as limiting the scope of the invention but as merely providing illustrations of some of the presently preferred embodiments of the present invention. Thus the scope of the present invention should be determined by the appended claims and their equivalents. 

What we claim is:
 1. A method for detecting from a digital chest image a ribcage boundary for computer-aided diagnosis with which anatomical structure information is detected from the digital chest image, including repetitions each comprising the steps of: deciding a size of a searching ROI mapped on the digital chest image, the searching ROI being deferent in position repetition by repetition; setting the searching ROI on the digital chest image; enhancing an image within the searching ROI set on the digital chest image; obtaining a profile of pixel values existing within the searching ROI; and analyzing the obtained profile so as to decide a candidate point for the ribcage boundary, wherein the deciding, setting, enhancing, obtaining, and analyzing Steps are repeated until a center of each searching ROI advances immediately before a given search limit, thereby a series of candidate points for the ribcage boundary being provided.
 2. The method of claim 1, wherein a step of lowering noise of the image residing in the searching ROI intervenes between the enhancing and obtaining steps.
 3. The method of claim 1, wherein a step of weighting the profile of pixel values intervenes between the obtaining and analyzing steps, wherein the profile consists of a horizontal pixel-value averaged vertical profile and a vertical pixel-value averaged horizontal profile and a weight of the weighting becomes small little by little as the weighting advances from a center of each of the vertical and horizontal the profiles toward an inside of lung fields.
 4. The method of claim 1, wherein the size of each of the plurality of searching ROIs mapped on the digital chest image through the repetitions are fixed and independent on locations of the searching ROIs.
 5. A system for detecting from a digital chest image a ribcage boundary for computer-aided diagnosis with which anatomical structure information is detected from the digital chest image, comprising: a landmark detector for detecting landmark information serving as a landmark in displaying a chest configuration depicted on the digital chest image; a candidate point series detector for detecting both of an upper ribcage boundary point series and right/left ribcage boundary candidate point series from image data of the digital chest image and the land mark information; a boundary point series combining unit for combining the upper ribcage boundary point series with the right/left ribcage boundary candidate point series so as to produce ribcage boundary information indicative of a contour of ribs of the chest; and an image displaying processor for displaying ribcage information made up of the landmark information and the ribcage boundary information on the image.
 6. The system of claim 5, further comprising means for weighting a pixel-value profile of an image residing within a searching ROI to be mapped on the digital chest image.
 7. The system of claim 6, further comprising means for smoothing the image residing within the searching ROI by using a moving average filter composed of a region in area smaller than the searching ROI.
 8. The system of claim 6, wherein the searching ROI is set to have a length larger than widths of the ribs and clavicles in the digital chest image, further comprising means for searching the points by using the searching ROI of which size is fixed from a search starting point to search end point. 